Human Performance question

[double_barrel]

Hi all, another daft question for your entertainment . In this case I know the 'correct' answer, but I wonder what the CAA thinks the answer is!

Capillary walls permit the passage of:

1. Cells
2. Gas
3. blood plasma
4. something silly I cannot recall

I know for sure that cells (macrophages and other immune cells) can and regularly do as part of their normal business pass the walls of a capillary. But I suspect that the CAA does not know this! So I could discount 1.
I know that small molecules including water, oxygen, glucose can pass - whether or not that means that 'blood plasma' can pass is surely debatable; some bits can some bits cannot. It does no pass through unaltered.
I know that gas should not exist at a capillary wall, only dissolved gasses, notable O2 and CO2 can and do freely pass. But not 'gas' unless you are in serious trouble.

I went for blood plasma on the grounds that it would also cover (dissolved) gas, but on reflection, I suspect that from a 'Human Performance' point of view, they are looking for gas as the answer since diffusion of CO2 and O2 is probably the thrust of the question.

So in case this question comes-up again, I wonder if anyone has come across it in a question bank and knows the expected answer.

Thanks